What we have in common is military training (10)
I believe the answer is:
similarity
'what we have in common' is the definition.
(how are similar)
'is military training' is the wordplay.
'training' indicates anagramming the letters (to train can mean to grow something into a shape).
'is'+'military'='ismilitary'
'ismilitary' with letters rearranged gives 'SIMILARITY'.
(Other definitions for similarity that I've seen before include "Resemblance" , "Likeness" .)